The present invention relates to a laser processing method for ablating a transparent material.
A wiring correcting device using a laser has contributed to the reduction of an LSI development procedure a great deal. To connect two wirings below an inter layer insulation film, the device forms via holes in the inter layer insulation film above the wirings by laser ablation and then connects the wirings by direct writing using a laser.
So long as the interlayer insulation film is formed of silicon nitride (SiN) or similar substance capable of absorbing a laser beam, a laser beam can implement desirable ablation with heat and damages an under wiring little. However, silicon dioxide (SiO2) or PSu (Phospho Silicate Glass) often used as an interlayer insulation film or a protection film is transparent for visible rays and therefore heated little itself even when irradiated by a laser beam lying in the visible range. In this case, ablation relies on a transmitted beam heating, underlying interconnections. However, this kind of processing is explosive and noticeably damages the under wiring.
To reduce the above damage to underlying interconnections ascribable to ablation, Japanese Patent Laid-Open Publication No. 1-296623 teaches a method which applies a thermal stress to a portion to be ablated by laser irradiation beforehand. Also, a method which causes ablation to occur with a UV (Ultra Violet) beam or a visible beam while exciting the atoms and molecules of a transparent material or photochemically decomposing a transparent material with a VUV (Vacuum UV) beam is discussed in Applied Physics, A64, pp. 477-481, 1997.
However, the problem with the method taught in the above Laid-Open Publication No. 1-296623 is that the influence of thermal strain and therefore damage is not avoidable although the processing threshold value can be reduced. The VUV scheme disclosed in Applied Physics has a problem that two beam sources, one for VUV and the other for UV, are essential and complicate the construction of an apparatus while increasing the size and cost of the apparatus. Moreover, optics capable of accurately patterning a VUV beam is not available at the present stage of development, so that the VUV irradiation area extends to portions not needing processing and aggravates damage.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 2-278865, 3-71991, 3-154385, 4-182093, 4-305392, 5-104278, and 8-85819.